The invention relates to the field of optical fiber systems involving connections between dissimilar optical fibers.
In addition to the standard communications fiber, a number of new types of optical fiber have been introduced into optical fiber based communication systems in recent years. Two groups of new fibres can be identified in terms of their refractive index distributions. The first group comprises dispersion controlling fibres with multiple layered refractive index profiles, while the second group comprises fibers that have a small core diameter but a high numerical aperture (NA) as compared with the standard communications fiber. The NA is defined as the square root of the difference between the squares of the refractive indices of the core and the cladding.
The standard communications single mode fiber is exemplified by the Corning product SMF 28 step index single mode fibre which consists of a circular core region of raised, approximately uniform, refractive index and a surrounding cladding region of uniform refractive index. The fiber consists of silica glass with the core doped with germania to give a raised refractive index and the cladding is typically undoped pure silica. The core diameter is about 9.0 xcexcm and the mode field diameter (MFD) is about 10 xcexcm at the wavelength of 1.55 xcexcm and the NA is about 0.1.
The dispersion controlling fiber (DCF) is exemplified by the Lucent Technologies DCF fiber which is used for dispersion compensation. This fiber has a multiple layered refractive index profile consisting of a raised refractive index core (doped with germania), surrounded by a ring layer of lowered refractive index (doped with fluorine), surrounded, in turn, by a slightly raised ring layer (doped with germania). The DCF fiber has a MFD of about 5.0 xcexcm at the wavelength of 1.55 xcexcm. The raising and lowering of the refractive index is with reference to the uniform silica cladding which surrounds the whole.
The high NA fibres are exemplified by the erbium doped fibre (e.g. Fibercore DF 1500F), the photosensitive fibre for Bragg grating writing (e.g. Fibercore PS1500) and the small core fibres used for pigtailing optoelectronic components (e.g. Fibercore SM1500).
In many potential applications, it is envisaged that these special optical fibers will be spliced permanently to standard communication fibers.
A standard method for connecting two lengths of standard communication fiber, referred to as fusion splicing, involves butting together the prepared ends of two fibers in the presence of a heat source e.g., a flame or electric arc such that the fiber ends melt and coalesce. Fusion splices are subject to optical losses, referred to collectively as xe2x80x9csplice loss.xe2x80x9d Various factors have been identified as contributing to splice loss, including lateral offset of the cores, differences in the optical characteristics of the mating fibers, and changes in the refractive index profile that take place during fusion.
When fibers having widely dissimilar mode field diameters (MFDs) and mode field shapes (MFSs) are spliced to one another, the mismatch of the mode fields at the location of the splice can result in high splice loss.
One technique for mitigating this contribution to the splice loss is described, for example, by D. B. Mortimore and J. V. Wright, xe2x80x9cLow-Loss Joints between Dissimilar Fibers by Tapering Fusion Splices,xe2x80x9d Electronics Letters, 22 (Mar. 13, 1986), pp. 318-319. This tapering technique involves first making a standard fusion splice and then drawing the softened glass in the vicinity of the splice such that the glass becomes constricted, decreasing the diameter of both the cladding and the core in the vicinity of the splice. This tapered region is said to function as a mode transformer that transforms the optical mode field of one fiber to that of the other with low optical loss. A standard communication fiber has reportedly been joined, with a total splice loss of 0.56 dB, to a fiber having a core diameter of 3.8 xcexcm and an MFD of 4.34 xcexcm. This tapering by drawing approach has never been practically demonstrated outside the laboratory.
An alternative approach to fusion splicing of fibers, based on the diffusion of dopants in the refractive index profile, was reported by, for example, W. Zell, et al., xe2x80x9cLow-Loss Fusion Splicing of PCVD-DFSM Fibers,xe2x80x9d Journal of Lightwave Technology, LT-5, (September 1987), pp. 1192-1195. The approach of Zell, et al. involves spreading the smaller of the cores of the (not very dissimilar) mating fibers by diffusing the index-raising dopant during an annealing step after the splice is formed. (The index-lowering dopant of the cladding was also found to diffuse during heating.). Zell, et al. reported that diffusion tapering was effective in reducing the optical loss in a fusion splice between a depressed cladding, single-mode (DCSM) fiber and a dispersion flattened, single-mode (DFSM) fiber having a smaller MFD than the DCSM fiber.
Significantly, the heat treatment, reported in that work, caused the concentrations of germanium and fluorine dopants, respectively, to exhibit a diffusion profile extending axially from the joint. At a wavelength of 1.3 xcexcm, a splice loss of 0.30 dB was achieved by Zell, et al. This splice loss was smaller than the theoretical loss in a step joint between the two fibers, and the difference was attributed to diffusion tapering. However, at a wavelength of 1.55 xcexcm, a somewhat greater loss, 0.35 dB, was observed, and no reduction of loss attributable to diffusion tapering was observed.
In a practical communication system, it is desirable for splices between different fibers to exhibit still smaller losses, e.g., losses much smaller than 0.3 dB. The Zell, et al. reference does not disclose a technique that can produce low-loss splices between fibers having drastically different core sizes, refractive index profiles, MFDs and MFSs. Indeed, at 1.55 xcexcm, which corresponds approximately to the operating wavelength of erbium amplifiers, Zell, et al. has failed to show any improvement in splice loss by diffusion tapering. Moreover, the improved splice reported there involved a pair of only moderately dissimilar fibers both with relatively large cores, i.e., fibers with respective MFDs of 10.1 xcexcm and 7.6 xcexcm at a wavelength of 1.55 xcexcm.
The work of Zell et el. has been extended by Cohen et al. (xe2x80x98Optical communications system comprising a fiber amplifierxe2x80x99 U.S. Pat. No. 5,074,633 Dec. 24, 1991). Cohen et al. describes a splice joint with a loss of less than 0.15 dB at 1.55 xcexcm between an erbium doped fiber with a MFD of less than 4 xcexcm and a communications fiber with a MFD of about 10 xcexcm. This result was also achieved with an annealing step after the splice was formed where the heat source was an oxy-hydrogen flame of about 0.6 mm in length.
Zell and Cohen each use diffusion tapering to a limited extent by preferentially diffusing the smaller of the cores at the annealing stage and keeping the diffusion of the larger of the cores to a minimum in an effort to equalise the core diameters. Thus, the diffusion takes place primarily in one fiber only. Cohen uses a maximum diffusion time of only 200 seconds at 2000xc2x0 C.
The difference in MFD and MFS between DCF and standard communications fiber is very large compared to the difference in fibers which have been spliced using the prior art. Thus, practitioners in the art have until now failed to provide a fusion splice that is capable, for operation at about 1.55 xcexcm, of joining a multiple layer fiber to a communications fiber having radically different MFDs and MFSs, with a total splice loss less than 0.2 dB over a wide wavelength range. Similarly, splice losses between the standard communications fibre and high NA fibres have not been demonstrated below about 0.13 dB.
Another problem arises in attempting to produce a low loss joint between fibers of different diameters such as a standard telecommunications fiber and a high NA fibers with small diameters as compared with the telecommunications fiber. Such small diameter high NA fibers are used in coils for gyroscopes and hydrophones. The large difference in diameter can give rise to significant losses at the splice.
The present invention provides an improved annealed optical fiber joint between first and second doped optical fibers fusion spliced to one another, having different core sizes and different refractive index profiles. According to the invention, a longitudinal diffused region is associated with the splice, comprising a length of both fibers, wherein the amount of diffusion increases as the splice is approached along each fiber, the length of the diffused region in each fiber being 3 mm or more; and the fusion splice has a total splice loss, over the range of signal wavelengths, of less than 0.2 dB.
The provision of a substantial diffused region in both fibers gives rise to a reduction in splice loss according to the invention. The length of the diffused region in each fiber may be approximately 5 mm.
The invention also includes a method of annealing an optical fiber joint between first and second optical fibers fusion spliced to one another for operation in a predetermined wavelength range, the second fiber having a different core size and different refractive index profile from the first fiber, characterised by heating the fibers in the region of the fusion splice to produce diffusion of dopants therein to form a longitudinal diffused region comprising a length of both fibers wherein the amount of diffusion increases as the splice is approached along each fiber, the length of the diffused region in each fiber being 3 mm or more; and the fusion splice has a total splice loss, over the range of signal wavelengths, of less than 0.2 dB.
According to the invention, the cores of the two fibers are both diffused substantially to reduce splice loss. The amount of diffusion in each of the fibers is much greater than in the aforementioned prior art. The diffusion times may be much longer, for example 3 to 30 minutes at a peak temperature of 2000xc2x0 C. The method according to the invention causes dopant diffusion in both of the fibers. However, for a good optical match at the joint is not necessary for the extent of the diffusion to be total. Indeed, a modest increase in the extent of the diffusion over that demonstrated in the prior art, provided that both fibers are exposed nearly equally to the diffusing heat source, can improve the joint losses considerably compared to the prior art.
In one example, one fiber is a standard step-index communication fiber, and the second fiber is a multilayered dispersion compensating fiber (DCF). A diffused dopant region is included adjacent the splice. The diameter of the communications fiber core increases gradually within the diffusion region as the splice joint is approached along this fiber. The diffusion of the various dopants in the DCF fiber also increases gradually as the splice joint is approached along this fibre. At the splice joint the diffusion of these dopants tend to cause the refractive index profile to converge optically to that of the diffused step index communications fiber. As a consequence of the diffusion region and its gradual longitudinal variation, the optical losses associated with the splice are relatively low, i.e. less than 0.2 dB at the operating wavelength, even when there is relatively high mismatch between the mode field diameters and mode field shapes (at a signal wavelength) in the respective fibers.
In a second example, one fiber is a standard step-index communication fiber, and the second fiber is a high NA fibre with a NA of 0.3. A diffused dopant region is included adjacent the splice. The diameter of both fiber cores increase gradually within the diffusion region as the splice joint is approached along the fibers. The diffusion of the core in the high NA fiber tends to cause it to converge optically to that of the diffused step index communications fiber. As a consequence of the diffusion region and its gradual longitudinal variation, the optical losses associated with the splice are relatively low i.e. less than 0.1 dB, even when there is relatively high mismatch between the mode field diameters (at a signal wavelength) in the respective fibers.
In another aspect, the invention provides a method of annealing an optical fiber joint between first and second optical fibers fusion spliced to one another for operation in a predetermined wavelength range, the first and second fibers having a different outer diameters, comprising heating and thereby softening the fibers in the region of the fusion splice, and axially forcing the spliced fibers towards one another to produce a fattening thereof in the region of the fusion splice.
This fattening technique according to the invention can ameliorate losses due to differences in optical fiber diameter but also can produce advantageous results with spliced fibers of the same diameter.
In a third example, one fiber is a standard step-index communication fiber with an outer diameter of 125 xcexcm, and the second fiber is a high NA fibre, with a cladding diameter of 80 xcexcm. A fattened region is produced in the region of the splice. The fattening is carried out so that diameters of both fiber cores increase gradually within the fattened region as the splice joint is approached along the fibers. A preferential fattening can be produced in the core in the high NA small diameter fiber which tends to cause it to converge optically to that of the fattened step index communications fiber. As a consequence of the fattened region and its gradual longitudinal variation, the optical losses associated with the splice are relatively low i.e. less than 0.1 dB, even when there is relatively high mismatch between the mode field and cladding diameters, at a signal wavelength, in the respective fibers.